This invention relates to an apparatus and method for detecting, measuring and correlating the oxygen and nitrogen content of a substance within an enclosed space and, in particular, for determining, with a high probability, whether such substance is an explosive.
There has been an increasing incidence of violent behavior throughout the world and this behavior has sometimes manifested itself by the placing of explosives in packages which are then sent through the mails or placed aboard aircraft. An urgent need exists for the better protection of Post Office employees, aircraft crews, passengers and others who may be killed or injured by the detonation of explosives. Explosives in aircraft baggage presents a particular hazard since their detonation while the aircraft is in the air is almost always catastrophic and such detonation may be caused by a change in pressure or temperature, time or by a radio or supersonic signal.
All modern explosives of which we are aware contain a relatively large amount of both nitrogen and oxygen, sufficient oxygen being required to very nearly oxidize all the carbon and hydrogen in the explosive. Examples of such explosives are nitroglycerin, sensitized nitromethane, trinitrotoluene, penterythritol tetranitrate, nitrocellulose, cyclonite, nitrostarch, nitrosugar and nitromannitol. Therefore, the detection of the presence of nitrogen and oxygen in high concentrations may be indicative of the presence of a bomb. More specifically, if a very high concentration of the element nitrogen is found in a suitcase or other enclosure where there is also a very high concentration of the element oxygen, there is a high probability the suitcase contains an explosive. Further, there are certain shapes which are more probable for bombs. Generally, the shape of a bomb is substantially cylindrical, the cylinder being several times as long as the distance across its base. A bundle of sticks of dynamite or nitroglycerine gelatins may be somewhat irregular in shape but, as a rule, it will be compact and located in only one part of the suitcase with the other contents of the suitcase distributed around the perimeter of the dynamite package. Compact shapes are usually necessary or the bomb may not explode in its entirety.
There are a number of non-explosive substances which have characteristics in common with explosive materials. These include cheese, leather, after shave lotion and acrylic plastics. For example, cheese contains nitrogen and, because of the moisture in it, has oxygen in combination with hydrogen in the form of water. In the pyrolysis of cheese the products obtained include water vapor, ammonia and amines, combustible organic vapors, elemental carbon and a relatively small amount of elemental nitrogen in gaseous form. On the other hand, nitroglycerine, the essential basis of many commonly available explosives, has different properties. Exploding under substantially ideal conditions, two moles of nitroglycerine yield three moles of diatomic nitrogen gas, six moles of carbon dioxide, five moles of water vapor, and half a mole of diatomic gaseous elemental oxygen in addition to a very large number of calories of heat energy.
Comparing cheese with nitroglycerine, it is seen that they differ not only in that the cheese won't explode, but in the fact that the explosive has a much higher population of nitrogen atoms per unit volume and concurrently a very high population of oxygen atoms in the same volume as compared to that in the same volume of cheese. With regard to after shave lotion, the glass in the bottle contains a large number of oxygen atoms per unit volume as does the enclosed liquid but it does not contain any nitrogen. The acrylic plastic has less oxygen and no nitrogen. Polyurethane plastics have nitrogen and oxygen but their total amount is far less than that of an explosive and, in particular, the amount of oxygen is less in the polyurethane plastic.